They can't allow you to withdraw what they don't
"physically" have. I suppose that "physically" isn't the exact word for cryptocurrencies, but you get what I mean. They had coins stolen so even though we see some balances in our accounts, they can't be put into the blockchain because Cryptopia doesn't hold those coins anymore.
As stated here:
https://twitter.com/Cryptopia_NZ/status/1102776289650335744 they'll offer "rebates"... now what those "rebates" mean, I don't know.
I dont have ERC20 or ETH Balance on my Cryptopia account and majority of my funds is on USDT and its not affected by the hack.
They should be allowed to be withdrawn while the users with affected coins can wait for their so called rebates.
USDT is also an ERC20 token, not just OMNI. and they have not made a public announcement of what has been lost, so no one knows.
Unless you can show on one or both of its blockchains, you cannot know if the USDT are safe. Have you checked on either of the blockchains for USDT to see if Cryptopia wallet has its coins?